1. Field of the Invention
The present invention relates to a one-input sense amplifier, arranged in an adder integrated circuit, for sensing a small signal of a carry signal transmitted from a lower digit to an upper digit.
2. Description of the Related Art
FIG. 1 is a one-input sense amplifier arranged in an adder integrated circuit to sense a carry signal.
This sense amplifier comprises an output npn bipolar transistor 43 having a base, emitter, and collector respectively connected to an input terminal 41, a power source for a ground voltage V.sub.SS, and an output terminal 42, a load resistor 44 connected between the output terminal 42 and a power source for a positive voltage V.sub.CC, a field effect transistor, e.g., a MOS transistor 45 having a source-drain path inserted between the input terminal 41 and the output terminal 42, and a MOS transistor 46 having a source-drain path inserted between the input terminal 41 and the voltage V.sub.CC.
In the sense amplifier having the above arrangement, the potential of a precharge carry signal from a lower digit is generally applied as an input to the input terminal 41. The potential at the input terminal 41 of the sense amplifier is set at a predetermined potential in an input waiting state for waiting for a carry signal input. During the sensing period of the carry signal, however, the potential at the input terminal 41 is kept at the precharge potential obtained in the input waiting state or gradually changes therefrom.
Assume that the potential of the input terminal 41 is set to be Hin (Hin.gtoreq.Vf where Vf is the threshold voltage of the base-emitter path of the bipolar transistor 43) in the input waiting state. At this time, the transistor 43 is set in an ON state, and a collector current flows therethrough, so that the potential at the output terminal 42 is set to be a low potential Lout determined by the ratio of the ON resistance of the transistor 43 to the resistance of the load resistor 44.
On the other hand, during the sensing period, when the potential of the input terminal 41 changes to be decreased to the low potential Lin lower than the threshold voltage Vf, the transistor 43 is turned off, and the collector current is cut off. At this time, the output terminal 42 is charged through the load resistor 44, and the output potential is set at a high potential Hout close to the voltage V.sub.CC.
Note that the MOS transistor 45 is controlled to be set in an ON state when the potential of the input terminal 41 returns from the low potential Lin to the high potential Hin. The input terminal 41 is then charged by the potential of the output terminal 42, and the input potential can be easily increased to the high potential Hin. Note that the MOS transistor 46 comprises a bias MOS transistor for supplying a current to the base of the output transistor 43.
In the conventional sense amplifier described above, since the output transistor 43 is kept on in the input waiting state, a through current flows across the power sources, thus increasing power consumption in the input waiting state. Since the output terminal is charged through the load resistor to obtain a potential different from that in the input waiting state, a high-speed sensing operation cannot be performed.